Scan conversion tube wherein the flood beam passes through the storage electrode and is scanned over an image dissector



Oct 25, 1 R. H. CLAYTON 3,281,622

SCAN CONVERSION TUBE WHEREIN THE FLOOD BEAM PASSES THROUGH THE STORAGE ELECTRODE AND IS SCANNED OVER AN IMAGE DISSECTOR Filed Feb. 18, 1964 5 Sheets-Sheet 1 INVENTOR. ROBERT H. CLAYTON BY 7M, W4

ATTORNEYS Oct. 25, 1966 R. H. CLAYTON 3,281,622

SCAN CONVERSION TUBE WHEREIN THE FLOOD BEAM PASSES THROUGH THE STORAGE ELECTRODE AND IS SCANNED OVER AN IMAGE DISSECTOR Filed Feb. 18, 1964 5 Sheets-Sheet 2 l 0 x 36 I 3 I4 a INVENTOR. ROBERT H. CLAYTON ATTORNEYS Oct. 25, 1966 R. H. CLAYTON 3,281,622

SCAN CONVERSION TUBE WHEREIN THE FLOOD BEAM PASSES. THROUGH THE STORAGE ELECTRODE AND IS SCANNED OVER AN IMAGE DISSECTOR Filed Feb. 18, 1964 5 Sheets-Sheet 5 INVENTOR. ROBERT H. CLAYTON ATTORNEYS United States Patent SCAN CONVERSION TUBE WHEREIN THE FLOOD BEAM PASSES THROUGH THE STORAGE ELEC- TRODE AND IS SCANNED OVER AN IMAGE DIS- SECTOR Robert H. Clayton, Fort Wayne, Ind., assignor to International Telephone and Telegraph Corporation, Nutley, N.J., a corporation of Maryland Filed Feb. 18, 1964, Ser. No. 345,785 14 Claims. (Cl. 31513) This invention relates generally to scan conversion tubes for converting an input video signal having a given scanning rate to an output video signal having a different scanning rate.

In one conventional form of scan conversion tube, an input video signal is used to modulate a writing pencil electron beam which is scanned over one side of an imperforate insulator at the scanning rate of the input signal thereby to write a charge pattern on the insulator corresponding to the input signal. A reading pencil electron beam is scanned over the opposite side of the insulator at the desired output scanning rate, the output video signal being extracted for example from a collector electrode which collects the secondary electrons emitted due to impingement of the reading beam on the insulator. Such scan conversion tubes together with other types of scan conversion tubes in general are characterized by poor resolution, write-through of the writing beam, i.e., crosstalk, and/or non-variable storage decay rates.

It is therefore an object of the invention to provide an improved scan conversion tube.

Another object of the invention is to provide an improved scan conversion tube having higher resolution than heretofore provided.

A further object of the invention is to provide an improved scan conversion'tube in which crosstalk is substantially eliminated.

One conventional form of signal-to-image display storage tube comprises a phosphor display screen, an apertured storage screen, a writing electron gun, and a flood electron gun. The storage screen conventionally comprises a fine mesh metal screen coated on the side facing the writing and flood gun with dielectric material having secondary emission properties. The writing gun directs a writing pencil electron beam toward the storage screen, the pencil beam being modulated in accordance with the input electrical signal and scanned over the dielectric surface of the storage screen thus writing a charge pattern on the storage screen corresponding to the input signal. The flood gun directs a low velocity flood beam of electrons toward the storage screen, the flood beam electrons passing through the apertures in the storage screen to the display screen being modulated by the incremental charges on the storage screen thereby providing a visible image on the display screen corresponding to charge pattern on the storage screen. In a conventional image dissector tube, an input optical image is focused onto an extended area photocathode which thus emits a flood beam of electrons corresponding to the optical image. This flood beam of electrons is then scanned over a defining aperture with the electrons which pass through the aperture being amplified by secondary emission electrodes and hence accelerated onto a target electrode thus developing an output video signal corresponding to the flood electron beam and in turn to the optical image. It will be thus be seen that in a conventional signal-to-image display storage tube, a flood beam of electrons modulated in accordance with an input video signal is provided whereas, in a conventional image dissector tube, a flood beam of electrons modulated in accordance with an input optical signal is converted to an output video signal.

In accordance with the invention therefore, a scan conversion tube is provided in which an input video signal is written onto an apertured charge storage screen by means of a writing pencil beam and in which a flood electron beam is passed through the charge storage screen being modulated by the incremental charges thereon, the thus-modulated flood beam then being scanned over an image dissector to provide the output video signal. In the preferred embodiment of the invention, an extended area photocathode and a flood illumination source are employed for providing the flood beam rather than the conventional thermionic flood gun with the Writing pencil beam passing through an aperture in the photocathode to provide concentric writing and flood beams which are simultaneously scanned over the storage screen thus permitting simultaneous writing and read-out. Crosstalk is eliminated by providing an electron velocity discriminator in the image dissector which deflects the electrons of the Writing beam which pass through the storage screen and the defining aperture of the image dissector away from the secondary emission accelerating electrodes. High resolution of both the writing beam and the modulated flood beam is provided by a solenoidal focusing coil extending throughout the full length of the tube which focuses the writing beam onto the storage screen and the modulated flood beam onto the defining aperture of the image dissector, scanning of the writing beam and flood beam in the writing section of the tube and of the modulated flood beam in the read-out section of the tube being provided by magnetic deflection coils.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompaying drawings, wherein:

FIG. 1 schematically illustrates one embodiment of the improved scan conversion tube of the invention;

FIG. 2 is a fragmentary schematic view showing a photocathode having twice the diameter of the storage screen with the flood beam undeflected;

FIG. 3 is a view similar to FIG. 2 showing the flood beam fully deflected;

FIG. 4 is a fragmentary schematic view showing a modification of the invention employing a curved photocathode surface and showing the flood beam fully deflected;

FIG. 5 is a fragmentary schematic view showing another modification of the invention employing a nonuniform focusing field and with the flood beam undeflected;

FIG. 6 is a fragmentary schematic illustration showing one form of velocity discriminator employed in the image dissector section of the invention; and FIG. 7 is a fragmentary schematic view showing another form of velocity discriminator.

Referring now to FIGS. 1, 2, and 3 of the drawing,

the preferred embodiment of the improved scan conversion tube of the invention, generally indicated at 10, comprises an axially-elongated evacuated envelope 11 having opposite ends 12, 13. A writing gun 14 is provided in the envelope 11 adjacent end 12 thereof and comprises a thermionic cathode sleeve 15 concentric with the longitudinal axis 16 of the tube and having a suitable heater 17 therein, a control grid 18 concentric with the tube axis, and an anode 19 likewise concentric with the tube axis. Heater 17 is connected to terminals 20 adapted to be connected to a suitable source of heater energizing potential, cathode 15 and anode 19 are respectively connected to terminals 22 and 23 for connection to suitable potentials, and control grid 18 is connected to input terminal 24 adapted to be connected to receive a time-based video input signal for modulating the pencil or writing electron beam which is provided by the writing gun 14 and directed toward the other end 13 of the tube.

An extended 'area photocathode is provided in front of the writing electron gun 14, photocathode 25 being concentric wit-h the tube axis 16 and normal thereto and having a central aperture 26 formed therein through which the writing electron beam passes. A suitable annular flood lamp 27 is provided surrounding or contained within envelope 11 forwardly of photocathode 25, the light provided by lamp 27 :passing through a transparent portion 28 of the wall of envelope 11 thus to illuminate the entire area of the photocathode 25, which in turn emits a low velocity flood beam of electrons toward end 13 of the tube.

A conventional charge storage screen 29 is positioned in the envelope 11 between ends 12, 13 and normal to the longitudinal axis 16 thereof. Storage screen 29 comprises a fine mesh metal backing screen 30 having its side which faces writing gun 14 coated with a suitable dielectric material having secondary emission properties, as at 32. A conventional collector screen 33is positioned in the envelope 11 between storage screen 29 and writing gun 14 [for collecting secondary electrons emitted from the dielectric coating 32 of the storage screen in response to impingement of the electrons of the writing beam thereon as is well known to those skilled in the art.

The backing screen 30 of the charge storage screen 29 is connected to terminal 44 for connection to suitable sources of writing and erasing potentials and the collector screen 33 is connected to terminal 45 for connection to a suitable :potential.

In order to provide planar accelerating equipotentials for the photocathode 25, a transparent conductive coating 34 is formed on the interior surface of the envelope 11 between the photocathode 25 and the collector screen 33, wall coating 34 being connected to terminal 35 adapted to be connected to a suitable potential.

It will now be seen that a pencil electron beam, shown by the dashed line 36 in FIGS. 2 and 3 is provided by the writing gun 14 and that a concentric flood beam, having lower electron velocity than the writing beam 36 is provided by the photocathode 25, as shown by the dashed lines 37.

In order to provide for deflection of both the writing electron beam 36 and the flood electron beam 37 and thus to scan both beams over the storage screen 29, vertical and horizontal magnetic deflection coils or yokes 38, 39 are respectively provided on the exterior of envelope 11 between photocathode 25 and the collector screen 33. Deflection yokes 38, 39 are connected to terminals 40, 42 for connection to suitable sources of sweep voltages to provide the type and rate of scanning corresponding to the scanning rate of the input video signal applied to the input terminal 24.

In order to permit simultaneous writing and read-out, it is necessary that the storage screen 29 at all times be immersed in the flood beam. Since the flood beam is V deflected by the deflection coils 38, 39 along with the writing beam 36, it will be seen that if the diameter of the photocathode 25 was the same as that of the storage screen 29, the storage screen would be fully immersed in the flood beam only at the midpoints of both vertical and horizontal deflection, i.e., only at that instant when the pencil beam 36 was directed at the exact center of the storage screen 29. Thus, in the illustrated embodiment incorporating a :planar flood cathode and a uniform magnetic focus field, and assuming balanced deflection, it is desirable to provide a flood cathode 25 having a diameter twice that of the useful area of the storage screen 29; reference to FIG. 3 will show that the excess diameter of photooathode 25 over that of the storage screen 29 assures that the entire storage screen 29 will 4 be flooded with the flood beam 37 for all positions of write beam deflection.

It will now be seen that when the writing beam 36 provided by the writing gun 14 is modulated by an input video signal applied to the input terminal 24, and when the thus-modulated writing beam 36 is scanned over the charge storage screen 29 by the deflection coils 68, 39, a charge pattern will be written onto the dielectric sunface 32 of the storage screen 29 corresponding to the input video signal. It will further be seen that the low velocity flood beam of electrons provided by the photocathode 25 will pass through the apertures in the storage screen 29 being modulated by the incremental charges thereon thus to provide in the read-out section 43 of the tube a modulated flood beam of electrons corresponding to the charge pattern written onto the storage screen 29.

In order to convert the modulated flood electron beam in the readout section 43 of the tube to an output video signal having a scanning rate diflering from that of the input video signal, an image dissector 46 is provided adjacent end 13 of the envelope 11. Image dissector 46 comprises an electrode 47 having a central defining aperture 48 concentric with the axis 16. On the side of aperture 48 remote from the storage screen 29 there is provided an electron velocity discriminator 49 in the form of an electrostatic mirror comprising a collector electrode 50 and a suppressor electrode 52. Collector electrode 50 and suppressor electrode 52 are respectively connected,

modulated flood beam electrons which pass through the aperture 48 are deflected by the suppressor screen 52 as shown at 56in FIG. 6.

A plurality of secondary emission accelerating elec trodes or dynodes 57 are provided respectively connected to terminals 58-61 for connection to progressively higher potentials. A target electrode 63 and an output electrode 64 are provided adjacent the final dynode 57, target electrode 63 being connected to terminal 65 for connection to a suitable source of potential and output electrode 64 being connected to terminal 66 by load resistor 67. Output terminal 68 is connected to the output electrode 64 by coupling capacitor 69. It will now be seen that the modulated flood electrons which passed through the defining aperture 48 are deflected by the velocity discriminator 49 to the initial dynode 57a, these modulated flood electrons being accelerated and amplified by secondary emission so as to provide an output signal across the load resistor 67.

In order to scan the modulated flood beam in the readout section 43 of the tube over the defining aperture 48 of the image dissector 46 thereby to provide a time-- based video output signal at the output terminal 68, vertical and horizontal magnetic deflection coils or yokes 70, 72 are :provided, on the exterior of envelope 11 between storage screen 29 and electrode 47 of the image dissector 46, deflection yokes 70, 72 being connected to terminals 73, 74 for connection to suitable sources of sweep voltages to provide the desired type and rate of scanning for the desired output video signal.

It will now be seen that at the output side of the storage screen 29, i.e., the side facing the read-out section 43, there are two (2) distinct currents. The first of these currents is the penetrating portion of the writing beam 36 which emerges from the storage screen 29 with a volt velocity determined by the potential of the writing gun cathode 15. This portion of the writing beam which passes through the apertures in the storage screen 29 is both modulated and scanned in time, however, its deflecting field provided by deflection coils 38, 39 does not penetrate into the read-out section 43 of the tube. The second current which emerges from the storage screen 29 is the modulated flood electron current. It will be seen that although the point of origin of the modulated flood electron current changes with its deflection in response to the writing deflection yokes 38, 39, the amplitude of the current at any given point in the read-out section 43 is determined only by the incremental charge on the corresponding area of the storage screen 29. It will thus be seen that beyond the storage screen 29 in the read-out section 43 of the tube, the extended-area low velocity modulated flood electron beam is like the flood electron beam of a photocathode which has its illumination corresponding to the charge pattern written onto the storage screen 29. y

In order to provide the requisite acceleration of the modulated flood beam from the storage screen 29 toward the defining aperture 48, while nevertheless permitting deflection of the modulated beam by the deflection yokes 70, 72, a field mesh screen electrode 75 is provided in the read-out section 43 of the envelope 11 closely spaced from the storage screen 29 and a tubular electrode or conductive wall coating 76 extends between the screen electrode 75 and the electrode 47. Screen electrode 75, wall coating 76 and electrode 47 are connected to terminals 78, 79, and 80 for connection to a suitable potential or potentials; in a specific embodiment of the invention, the same potential is applied to the screen electrode 75, wall coating 76 and electrode 47 and thus these elements may be connected together within envelope 11 and connected to a single external termina It will be readily seen that with the field mesh screen 75, Wall coating 76 and electrode 47 connected to the same potential which is somewhat elevated with respect to the potential of the backing screen 30 of the storage screen 29, the modulated flood beam of electrons will be accelerated in the zone between the storage screen 29 and the field mesh screen 75, thus leaving the zone between the field mesh screen 75 and the electrode 47 substantially field-free to permit ready deflection of the modulated flood beam by the deflection yokes 70, '72.

In order to focus the writing beam 36 onto the storage screen 29 and to focus the modulated flood beam onto the electrode 47 of the image dissector 46 thereby to improve the resolution of the tube, the entire writing section 82 and the entire read-out section 43 of the tube are immersed in an'axial solenoidal magnetic field produced by a solenoid focusing coil 83 surrounding the tube and the deflection yokes 38, 39 and 70, 72, and extending substantially between the ends 12, 13 of the tube. Solenoid focus coil 83 is connected to terminals 84 for connection to a suitable source of direct current energizing potential. The strength of the magnetic field-produced by the solenoid coil 83 and the distance from the aperture 26 in the photocathode 25 to the plane of the dielectric surface 32 on the storage screen 29 are chosen so that the writing beam 36 provided by the writing gun 14 describes an integral number of full loops of focus between aperture 26 and dielectric surface 32. The strength of the magnetic field provided by the solenoid coil 83 and the distance between the plane of the metal backing screen 30 of the storage screen 29 and the electrode 47 are also correlated so that the modulated flood beam describes an integral number of full loops of focus between metal backing screen 30 and electrode 47. The potential of the writing gun cathode is chosen to yield a satisfactory secondary emission ratio at the dielectric surface 32 of the storage screen 29 with acceptable writing speed but nevertheless to provide a relatively low velocity writing beam so as to permit ready deflection thereof by the deflection yokes 38, 39 in the presence of the axial magnetic focusing field provided by the solenoid focusing coil 83; in conventional signal-to-image display storage tubes, a high velocity stiff writing beam is employed in order to provide the desired resolution, however, in the tube of the invention by virtue of the focusing of the writing beam provided by the axial magnetic focusing field, the velocity of the writing beam can be substantially reduced thereby to permit deflection by the magnetic deflection yokes 38, 39. It will be readily seen that virtually the entire acceleration of the Writing beam 36 takes place prior to its passage through the aperture 26 in the photocathode 25, the wall coating 34 in the writing section 82 of the tube providing a virtually field-free zone to facilitate deflection of the writing beam by the deflection yokes 38, 39. A small potential difference is, however, provided between the photocathode 25 and the wall coating 34 in collector screen 33, in order to provide acceleration for the photoelectrons which form the flood beam 37 toward the storage screen 29, this potential difference being selected after focusing of the writing beam on the storage screen has been achieved in order to defocus the flood beam, i.e., the image of the photocathode 25 at the dielectric surface 32. This serves to eliminate the dead spot in the photocathode aperture 26. Furthermore, since the flood beam is scanned over the entire area of the storage screen 29 by deflection yokes 38, 39 any dimming of the illumination of the storage screen 29 by the flood beam due to the aperture 26 in the photocathode will be time shared by the entire scene, i.e., the entire area of the storage screen 29, thus further reducing the effect of such dimming.

In a conventional signal-to-i-mage display storage tube, each aperture in the storage screen in essence forms an individual electron lens which focuses the flood beam electrons passing therethrough in a plane very close to the plane of the storage screen with the result that the flood electrons subsequently diverge to a large circle of confusion at the phosphor display screen, thus adversely affecting the resolution of the displayed image. With the axial magnetic focusing in the read-out section 43 of the tube of the invention provided by the solenoid focusing coil 83, the modulated flood beam electrons are reconverged or refocused onto the plane of the electrode 47 of the image dissector 46 in which the defining aperture 48 is located, thus to improve the resolution of the output signal. Likewise, in the usual electrostatically focused writing electron gun employed in conventional signal-toimage display storage tubes, the object-tolens distance is significantly less than the lens-to-storage screen distance which gives rise to a magnification of the spot produced on the storage screen by the writing beam. In the improved tube of the invention, however, the solenoidal focusing field in the writing section 82 provided by the solenoid focus coil 83 provides a magnification of unity, thus providing high resolution of the charge pattern written onto the dielectric surface 32 of the storage screen 29 by the writing beam.

In a specific embodiment of the improvedscan conversion tube shown in FIG. 1 in which the solenoid focus coil 83 is energized to provide a field strength of 60 g-auss, satisfactory operation may be provided by the application of the following potentials:

Writing gun cathode 15 800 volts Writing gun control grid 18 800 to 850 volts Writing gun anode 19 Ground. Photocathode 25 Ground.

Wall coating 34 200 volts Collector screen 33 200 volts 10 volts (writing).

Backin cr 30 g 5 10 volts (erase).

Erasure of the stored charge image on the dielectric surface 32 of the storage screen 29 may be accomplished during the active scanning time by the application of erase pulses having a repetition rate which is some multiple of the scanning frequency of the writing beam, or by the application of a continuous erase pulse during retrace.

Referring now to FIG. 4 in which like elements are indicated by like reference numerals, by curving the photocathode surface along radii extending toward the storage screen 29, the extent of the photocathode surface perpendicular to the axis of the tube may be reduced, as shown at 85 in FIG. 4. With this arrangement, it is not necessary that the diameter of the envelope 11 in the region of the photocathode be twice that required in the region of the storage screen as in the embodiment of FIG. 1.

Referring now to FIG. in which like elements are still indicated by like reference numerals, a planar photocathode 86 having the same diameter as the storage screen 29 may be provided by employing a non-uniform focus field. Here, the strength of the focusing field in the area of writing section 82 nearest cathode 25 would be greater than that provided nearer mesh 33, and also in section 82 the focusing field in the writing section 82 preferably being tapered as shown in FIG. 5. With the employment of a tapered focusing field in the writing section of the tube, the ratio of the area of photocathode 86 to the area of the storage screen 29 is set by the ratio of the flux densities of the focusing field associated with each.v

Referring now to FIG. 7, the electrons of the modulated flood beam which pass through the defining aperture 48 in the electrode 47 of the image dissector 46 may be deflected to the first dynode 57a by means of a strong magnetic field transverse to the axis of the tube, as shown at 87 in FIG. 7. It will be readily apparent that other forms of electron velocity discriminators may be employed for deflecting the relatively higher velocity writing beam electrons which pass through the aperture 48 away from the first dynode 57a and/ or deflecting the relatively lower velocity modulated flood beam electrons which pass through the defining aperture 48 to the first dynode.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention.

What is claimed is:

1. A scan conversion tube comprising: apertured charge storage screen means; means for forming and directing pencil and flood electron beams toward said storage screen means; first deflection means for scanning at least said pencil beam over said storage screen means; means for modulating said pencil beam in accordance with an input signal whereby said pencil beam writes a charge pattern on said storage screen means corresponding to said-input signal and said flood beam passes through the apertures in said storage screen means being modulated by said charge pattern; electron image dissector means on the side of said storage screen means remote from said beam forming and directing means and including means defining an aperture and output circuit means; and second deflection means for scanning said modulated flood beam over said aperture thereby developing an input signal in said output circuit corresponding to said charge pattern.

2. The tube of claim 1 wherein said image dissector means includes electron velocity discriminating means for separating the electrons of said pencil beam from the electrons of said flood beam whereby only the electrons of said modulated flood beam reach said output circuit.

3. The tube of claim 1 wherein said pencil and flood beams are concentric, and wherein said first deflection means acts upon both of said beams.

4. The tube of claim 3 wherein both of said deflection means are magnetic and further comprising means for providing a magnetic field extending axially between said sector means for focusing said pencil beam between said beam forming and directing means and said storage screen means and said flood beam between said storage screen means and said image dissector means.

5. A scan conversion tube comprising: an evacuated envelope having a longitudinal axis and spaced opposite ends; first electron gun means in said envelope at one end thereof and concentric with said axis for forming and directing a pencil electron beam toward the other end of said envelope; second electron gun means in said envelope at said one end thereof normal to and concentric With said axis for forming and directing a flood electron beam toward said other end of said envelope; apertured charge storage screen means in said envelope between said ends and normal to said axis; first magnetic deflection means acting upon both of said beams for scanning the same over said storage screen means; means for modulating said pencil beam in accordance with an input signal whereby said pencil beam writes a charge pattern on said storage screen means corresponding to said input signal and said flood beam passes through the apertures in said storage screen means being modulated by said charge pattern; electron image dissector means in said envelope at said other end thereof and including means defining an aperture concentric with said axis, and output circuit means; and second magnetic deflection means for scanning said modulated flood beam over said defining aperture thereby developing an output signal in said output circuit corresponding to said charge pattern.

6. The tube of claim 5 wherein said second electron gun includes a planar cathode normal to said axis and having an aperture therein concentric with said axis through which said pencil beam passes.

7. The tube of claim 6 wherein said cathode has an area at least twice the area of said storage screen means.

8. The tube of claim 6 wherein said cathode is a photocathode, and further comprising means for illuminating said cathode thereby to provide said flood beam.

9. The tube of claim 5 wherein said second electron gun includes an extended area cathode having an aperture therein concentric with said axis through which said pencil beam passes, said catlhode being curved toward said storage screen means.

10. The tube of claim 5 further comprising a solenoid coil surrounding said envelope and extending substantially between the ends thereof for providing a solenoidal magnetic field extending axially through said envelope and parallel with said axis for focusing said pencil beam between said first electron gun means and said storage screen means and said flood beam between said storage screen means and said image dissector means.

11. The tube of claim 5 wherein said second electron gun includes a planar cathode normal to said axis and having an aperture therein concentric with said axis through which said pencil beam passes and means for providing a solenoidal magnetic field extending axially through said envelope between said ends thereof which field is weakest adjacent said second electron gun means and tapers to a uniform field extending from a point intermediate said second electron gun means and said storage screen means to said image dissector means, said magnetic field focusing said pencil beam between said first electron gun means and said storage screen means and said flood beam between said storage screen means and said image dissector means.

12. The tube of claim 5 further comprising a screen electrode closely spaced from said storage screen on the side thereof facing said image dissector means, and a tubular electrode extending between said screen electrode and said image dissector means and concentric with said axis, said second deflection means being positioned between said screen electrode and image dissector means.

13. The tube of claim 12 wherein said image dissector means includes secondary emission electrode means for accelerating the electrons of said flood beam which pass through said defining aperture, and electron velocity discriminator means for deflecting the electrons of said pencil beam which pass through said defining aperture away from said secondary emission electrode means whereby only the electrons of said modulated flood beam reach said secondary emission electrode means.

14. A sc-an conversion tube comprising: an evacuated envelope having a longitudinal axis and spaced opposite ends; electron gun means in said envelope at one end thereof and concentric with said axis for forming and directing a pencil electron beam toward the other end of said envelope; and extended area photocathode having an aperture therein concentric with said axis through,

which said pencil beam passes; means for illuminating said photocathode thereby to direct a flood electron beam toward said other end of said envelope, said flood beam being concentric with said pencil beam; apertured charge storage screen means in said envelope intermediate the ends thereof and normal to said axis; collector electrode means spaced from said storage screen means on the side thereof toward said one end of said envelope; first tubular electrode means extending between said collector electrode means and said photocathode and concentric with said axis for establishing planar accelerating equipotentials for said flood beam; first magnetic deflection means between said photocathode and said collector electrode means and acting upon both of said beams for scanning the same over said storage screen means; means for modulating said pencil beam in accordance with an input signal whereby said pencil beam writes a charge pattern onto said storage screen means corresponding to said input signal and said flood beam passes through the apertures in said storage screen means being modulated by said charge pattern; electron image dissector means in said envelope at the other end thereof, said image dissector means com- 10 prising means defining an aperture concentric with said axis, secondary emission electrode means for accelerating the electrons of said modulated flood beam which pass through said defining aperture, output circuit means for receiving the accelerated electrons and providing an output signal in response thereto, and electron velocity discriminator means for deflecting the electrons of said pencil beam which pass through said defining aperture away from said secondary emission electrode means; means for accelerating said modulated flood beam in a zone adjacent said storage screen means and for providing a field-free space between said zone and said image dissector means comprising a screen electrode closely spaced from said storage screen means on the side thereof facing said image dissector mean and second tubular electrode means extending between said screen electrode and said image dissector means and concentric with said axis; second magnetic deflection means between said screen electrode means and said image dissector means for scanning said modulated flood beam over said defining aperture thereby developing an output signal in said output circuit corresponding to said charge pattern; and a solenoid coil surrounding said envelope and extending substantially between the ends thereof for providing a solenoidal magnetic field extending axially through said envelope for focusing said pencil beam between said electron gun means and said storage screen means and said flood beam between said storage screen means and said image dissector means.

References Cited by the Examiner UNITED STATES PATENTS 2,743,378 4/1956 Covely 315--13 X DAVID G. REDtINBAUGH, Primary Examiner. T. A. GALLAGHER, Assistant Examiner. 

1. A SCAN CONVERSION TUBE COMPRISING: APERTURED CHARGE STORAGE SCREEN MEANS; MEANS FOR FORMING AND DIRECTING PENCIL AND FLOOD ELECTRON BEAMS TOWARD SAID STORAGE SCREEN MEANS; FIRST DEFLECTION MEANS FOR SCANNING AT LEAST SAID PENCIL BEAM OVER SAID STORAGE SCREEN MEANS; MEANS FOR MODULATING SAID PENCIL BEAM IN ACCORDANCE WITH AN INPUT SIGNAL WHEREBY SAID PENCIL BEAM WRITES A CHARGE PATTERN ON SAID STORAGE SCREEN MEANS CORRESPONDING TO SAID INPUT SIGNAL AND SAID FLOOD BEAM PASSES THROUGH THE APERTURES IN SAID STORAGE SCREEN MEANS BEING MODULATED BY SAID CHARGE PATTERN; ELECTRON IMAGE DISSECTOR MEANS ON THE SIDE OF SAID STORAGE SCREEN MEANS REMOTE FROM SAID BEAM FORMING AND DIRECTING MEANS AND INCLUDING MEANS DEFINING AN APERTURE AND OUTPUT CIRCUIT MEANS; AND SECOND DEFLECTION MEANS FOR SCANNING SAID MODULATED FLOOD BEAM OVER SAID APERTURE THEREBY DEVELOPING AN INPUT SIGNAL IN SAID OUTPUT CIRCUIT CORRESPONDING TO SAID CHARGE PATTERN. 